1. Field of the Invention
The present invention relates to foamed cementing operations in subterranean zones, and more particularly, to foamed well cement compositions having improved mechanical properties and methods of using the compositions in subterranean well cementing operations.
2. Description of the Prior Art
Hydraulic cement compositions are commonly utilized in subterranean operations, particularly subterranean well completion and remedial operations. For example, hydraulic cement compositions are used in primary cementing operations whereby pipe strings such as casings and liners are cemented in well bores. In performing primary cementing, hydraulic cement compositions are pumped into the annular space between the walls of a well bore and the exterior surface of the pipe string disposed therein. The cement composition is permitted to set in the annular space, thereby forming an annular sheath of hardened substantially impermeable cement therein that substantially supports and positions the pipe string in the well bore and bonds the exterior surfaces of the pipe string to the walls of the well bore. Hydraulic cement compositions also are used in remedial cementing operations such as plugging highly permeable zones or fractures in well bores, plugging cracks in holes in pipe strings, and the like.
Cement compositions utilized in well applications must often be lightweight to prevent excessive hydrostatic pressure from being exerted on subterranean formations penetrated by the well bore whereby the formations are unintentionally fractured. Thus, foamed cement compositions are often used in subterranean well applications. In addition to being lightweight, a foamed cement composition contains compressed gas which improves the ability of the composition to maintain pressure and prevent the flow of formation fluids into and through the cement composition during its transition time, i.e., the time during which the cement composition changes from a true fluid to a hard set mass. Foamed cement compositions are also advantageous because they have low fluid loss properties. Additionally, foamed cements have a lower modulus of elasticity than non-foamed cements, which is desirable as it enables the cement, inter alia, to resist hoop stresses when the cement encases pipe that expands from internal pressures.
A stable foamed cement may be generated in situ in circumstances such as when the cement composition contains an expanding additive, such as a fine aluminum powder, that generates a gas within the composition as it reacts with the high pH of the cement slurry. In other cases, a stable foamed cement may be generated when Portland cement, or any other hydraulic cement, has air or a compressed gas such as nitrogen injected with proper surfactants.
Set cement in subterranean formations, and particularly the set cement sheath in the annulus of a well bore, may fail due to, inter alia, shear and compressional stresses exerted on the set cement. This may be particularly problematic in hostile subterranean formations. In these types of formations, set cements often fail as a result of the stresses exerted on the set cement.
The stress exerted on the cement as referred to herein means the force applied over an area resulting from the strain caused by the incremental change of a body's length or volume. The stress is generally thought to be related to strain by a proportionality constant known as Young's Modulus. Young's Modulus is known to characterize the flexibility of a material.
There are several stressful conditions that have been associated with well bore cement failures. One example of such a condition results from the relatively high fluid pressures and/or temperatures inside of the set casing during testing, perforation, fluid injection, or fluid production. If the pressure and/or temperature inside the pipe increase, the resultant internal pressure expands the pipe, both radially and longitudinally. This expansion places stress on the cement surrounding the casing causing it to crack, or the bond between the outside surface of the pipe and the cement sheath to fail in the form of, inter alia, loss of hydraulic seal. Another example of such a stressful condition is where the fluids trapped in a cement sheath thermally expand causing high pressures within the sheath itself. This condition often occurs as a result of high temperature differentials created during production or injection of high temperature fluids through the well bore, e.g., wells subjected to steam recovery processes or the production of hot formation fluids. Other stressful conditions that can lead to cement failures include the forces exerted by shifts in the subterranean formations surrounding the well bore or other over-burdened pressures.
Failure of cement within the well bore can result in radial or circumferential cracking of the cement as well as a breakdown of the bonds between the cement and the pipe or between the cement sheath and the surrounding subterranean formations. Such failures can result in at least lost production, environmental pollution, hazardous rig operations, and/or hazardous production operations. A common result is the undesirable presence of pressure at the well head in the form of trapped gas between casing strings. Additionally, cement failures can be particularly problematic in multi-lateral wells, which include vertical or deviated (including horizontal) principal well bores having one or more ancillary, laterally extending well bores connected thereto.
In both conventional single bore wells and multi-lateral wells having several bores, the cement composition utilized for cementing casing or liners in the well bores must develop high bond strength after setting and also have sufficient resiliency, e.g., elasticity and ductility, to resist loss of pipe or formation bonding, cracking and/or shattering as a result of all of the stressful conditions that may plague the well, including impacts and/or shocks generated by drilling and other well operations.
Because a typical foamed cement composition will have a lower tensile strength than typical non-foamed cement, a foamed cement is more susceptible to these stressful conditions. As a result, foamed cements may not be as useful in subterranean applications.